Polymer materials have been instrumental in improving the health and living conditions of the world's growing populations by providing cheap and durable materials for such varied applications as highway and building construction, water purification systems, medical applications and food packaging. The major end users of polymeric materials in Canada are plastic manufacturers with the polymer cost accounting for 30 to 50% of the final value on a plastic product. The Canadian industry for plastics involving 1200 companies was valued at US $5.56 billion in 1997 with Canadian producers exporting 60% of their polymer products. According to a report prepared in 2000 by the Canadian Plastics Producers, the Canadian and worldwide demand for plastic products is expected to grow faster than the economy as a whole, with an annual growth rate of 5 to 7% worldwide until 2008. Fueling that growth, it is stated, will be products with improved performance. As we move into the 21st century the performance demands on polymers are increasing as they are applied to ever more demanding and sophisticated applications, in particular, microelectronics and nanotechnology. The challenge for the polymer chemist is to develop new understanding and insight into existing or new technologies to enable the synthesis of these materials in a predictable, precise and cost-efficient manner.
In the last few decades, polymer chemistry has evolved from a physical science, in which the focus has been on measuring physical properties and studying kinetic issues on commercially available polymers, to a synthetic science where the creative design and synthesis of precision materials is the focus. The key means of achieving control over polymer properties is to control the molecular weight (MW) and polydispersity (PD) of the polymerization process. Living polymerization processes offer precisely this potential. In these processes, chain transfer and termination reactions that are endemic to most conventional polymerizations are reduced. This allows polymer growth to occur with excellent MW control and very low polydispersity indices (PDIs), and also permits the synthesis of block copolymers and polymer materials with complex macromolecular architecture. There are several different kinds of living polymerization processes, all with specific strengths as well as challenges for their exploitation for commercial impact.
A. Living Radical Polymerization
Living-radical polymerization is a relatively new class of living polymerization that has great potential for eventual commercial impact. Living-radical polymerization is positioned to be successful because (i) it is built on the foundation of the conventional radical polymerization process, which is already commercially successful and dominant, (ii) it is economical to perform, (iii) it can provide the control that is required for the new generation of polymeric materials and (iv) the infrastructure required to do these polymerizations is already in place in industry and the transition of living-radical polymerization to an industrial setting can be accomplished with minimal extra-capital expenditure.
The basic premise for living radical polymerization processes is outlined in simplified form in FIG. 1. Under conventional radical polymerization conditions, the growing reactive polymer chains Pn. are subject to various bimolecular termination and chain transfer reactions. However, the introduction of a terminating species T. (typically at a few mol % level) which can react quickly and reversibly with Pn. sets up an equilibrium between dissociated (“active”) polymer radical and polymer bound to T. (“dormant” form, Pn.T) which cannot react with monomer. The concentration of active polymer P. is controlled such that bimolecular chain/termination reactions are minimized, while reactions with monomer (propagation) can still occur at appreciable rates. These features convert the conventional polymerization process into one displaying the hallmarks of a living polymerization—linear growth in molecular weight versus monomer conversion, low PDI (<1.5), and the ability to perform chain extension reactions and block copolymer synthesis. As a result, living radical polymerization has become one of the most intensively studied areas in polymer synthesis in the past several years.
Currently there are three major classes of living-radical polymerization processes: Atom Transfer Radical Polymerization (ATRP), Reverse Addition Fragmentation Chain Transfer polymerization (RAFT), and Stable Free Radical Polymerization (SFRP). While there are mechanistic subtleties that distinguish all three processes, the first two share a common feature in that the terminating species T. in FIG. 1 is not a “free” radical when dissociated from Pn.; rather, T. is reversibly transferred between Pn. and another species (transition metal species in the case of ATRP and organosulfur compounds in RAFT). In SFRP the dissociated species T. is a stable radical. All three processes have different strengths and weaknesses, but at present each of the major living-radical systems has outstanding issues which currently limit their commercial viability.
Among the three living radical systems, SFRP stands out as an attractive process with considerable commercial potential because (a) it does not suffer from metal contamination of the polymer (as do ATRP derived polymers) and (b) SFRP-based processes are considerably more robust and less capricious than RAFT polymerizations. Effective SFRP processes were first reported in 1993, when it was demonstrated that high molecular weight (MW) polymers could be synthesized with molecular weight distributions (MWDs) narrower than what was considered theoretically possible at the time for a free radical polymerization process. The key to the SFRP process is the use of a stable free radical which does not initiate the polymerization but can reversibly terminate growing polymer chains. In a typical polymerization (FIG. 1), an initiator molecule initiates the growth of the polymer chains (Pa), which very quickly react with T.—in this case, TEMPO (FIG. 2), a stable nitroxide radical—to give dormant TEMPO-terminated polymer chains (Pa.T). As heating is continued, the relatively weak bond between the polymer chains and TEMPO breaks. This releases reactive polymer chains with free radical functionality on the terminal monomer unit. The reactive polymer chains then react with more monomer and increase in length. At some point the growing polymer chains react again with TEMPO to form longer TEMPO-terminated polymer chains (Pb.T). This cycle repeats itself until the monomer is consumed. Thus, as long as there is monomer (M) present, the polymer chains continue to grow in a controlled fashion where the molecular weight of the polymer is predicted by the amount of chains initiated and the amount of monomer used.
Several stable radical species have been explored for SFRP purposes, but by far the dominant class of radicals that have been studied are the nitroxides R2NO., typified by TEMPO. Nitroxides have been subjected to intense studies which have led to a better understanding of the specifics of the nitroxide-mediated SFRP process as well as progress in the efficacy of these systems. However there is still significant room for improvement. For example, nitroxide-mediated SFRP works very well for polystyrene production, but in the important class of acrylate- and methacrylate-based polymers there are still outstanding issues. Specific nitroxides have been developed that can mediate SFRP of some acrylates, but these nitroxides are very difficult to prepare and handle because of their instability. To date nitroxides have not been demonstrated to successfully mediate the SFRP of methacrylates.
In U.S. Pat. No. 6,114,499 discloses a stable free radical mediated polymerization process that provide homopolymer and copolymer resin products that possess narrow polydispersity properties and a high monomer to polymer conversion. In particular, U.S. Pat. No. 6,114,499 relates to stable free radical mediated or pseudoliving polymerization processes that yield branched homopolymers and copolymers having number average molecular weights (Mn) above about 100 to about 200,000 and having a polydispersity ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of from about 1.0 to about 2.0. While the results disclosed are specific to nitroxides, examples of suitable and preferred stable free radicals are disclosed in U.S. Pat. No. 6,114,499 by reference to U.S. Pat. No. 3,600,169. These include: nitrogen-centered stable free radical such as organic hydrazyls, verdazyls, and pyridinyl compounds; non-nitroxide oxygen centered stable free radicals such as aroxyls and the like; and carbon centered stable free radicals such as aryl alkyls and aryl cycloalkyls with the unpaired electron residing on a carbon atom in the alkyl or cycloalkyl substituents.
U.S. Pat. No. 6,068,688 discloses modified particles for use in living free radical polymerization. The stable free radicals contemplated in that patent include nitroxide free radicals such as 2,2,5,5-tetramethyl-pyrrolidinyloxy and 2,2,6,6-tetramethyl-piperindinyloxy, organic hydrazyl compounds, organic verdazyl compounds, organic aroxyl compounds (e.g., 2,4,6 tri-tertiary butyl phenoxy radical, gaivinoxyl (2,6 ditertiary butyl alpha 3,5 ditertiary butyl oxo 2,5 cyclohexadiene-1 ylidene para tolyoxy) radical), aryl alkyl or aryl cycloalkyl where the unpaired electron is on a carbon atom, substituted triphenyl methyl, substituted triphenyl amine, and derivatives of these compounds. The polymer synthesized using the modified particles is necessarily attached to the particle, and hence is not free standing.
As noted above, verdazyls have been suggested for use in SFRP. This is because they have been identified as stable free radicals. However, work on one class of these, the type 1 verdazyls, (see FIG. 3) has shown that they were not very effective in controlling styrene polymerization. This may indicate that the assumed relationship between stable free radicals and superior reduction in chain termination during polymerization is erroneous. Certainly, Ananchenko et al. (Ananchenko, G. S.; Souaille, M.; Fischer, H.; Mercier, C. L.; Tordo, P, J. Polym. Sci., Part A: Polym. Chem., 2002, 40, 3264-3283) argue that the rates of polymerization are controlled by the bond dissociation energy of the bond between the stable free radical moiety and the end of the propagating polymer chain and therefore, suggesting a mechanism that is not reliant on the stability of the stable free radical.
In support of this notion, it is known that during the course of SFRP reactions using nitroxide, small amounts of termination reactions occur which gradually and irreversibly consume active polymer chains. This leads to a buildup of excess radical (nitroxide) which shifts the equilibria in FIG. 1 to the dormant side, thereby shutting down the reaction. There have been several approaches to circumventing this for nitroxides by using additives that react with the excess nitroxide. These additives serve to destroy the nitroxide. Despite this, there has, to our knowledge, not been attempts to identify and use inherently moderately unstable free radicals in SFRP reactions.
It is an objective of the disclosed embodiment to overcome the deficiencies in the prior art.